It has been known for a long time that gasoline product of desirable octane can be obtained from selected hydrocarbon fractions by catalytic cracking. However, the yield of such desired gasoline products varies considerably with the composition of the oil feed charged to the cracking operation as well as the severity of the operating conditions employed. It is further known that heavy oils such as residual oils have a large percentage of very refractory components which are more difficult to crack and, in general, cause excessive amounts of coke to be deposited on the catalyst. Furthermore, metal contaminants in a heavy oil feed poison and inactivate the catalyst. Therefore, in the prior art, these undesirable components in the oil feed have been reduced by techniques such as hydrogenation, thermocracking, and/or adsorption on absorbent particle material of little or no cracking activity for the recovery of a more suitable oil feed. In this connection, mild thermal cracking and visbreaking operations, with or without the presence of hydrogen, have been relied upon to produce a more desirable feed material for conversion by catalytic cracking to desired gasoline and/or light fuel oil products.
Residual oil, coker gas oils and other materials high in polynuclear aromatics are known as distress stocks in the petroleum industry. These oils are, therefore, often sold in fuel oil blends or thermally processed, as recited above, to obtain lighter, more desirable components. Residual oils contain large quantities of components, having coke forming tendencies as well as metal contaminants which adversely affect the stability and activity of modern-day cracking catalysts. Coker gas oils high in polynuclear aromatics generally low in metal contaminants also are coke formers and generally considered to be poor cracking feed stocks.
The utilization of relatively high activity catalysts comprising high activity crystalline zeolite cracking catalysts has been responsible for developing refinements in cracking technology or techniques to reduce catalyst inventory systems and to more effectively take advantage of the catalyst activity, selectivity and its operating sensitivity. Reducing the size of equipment and catalyst inventory contributes to an economic advantage readily accepted by the industry.
The following U.S. patents have been considered in the preparation of this application; U.S. Pat. Nos. 3,904,548; 2,994,659; 3,158,562; 3,193,494; 3,896,024; 3,894,936; 3,886,060; 3,856,659; and 3,847,793.
The present invention is concerned with the use of a low catalyst inventory, riser cracking operation using high activity crystalline zeolite catalyst to effect a selective conversion of hydrocarbons varying considerably in chemical and physical composition characteristics. More particularly, the present invention is concerned with disposing of distress stocks such as coker gas oils by fluid cracking.